This invention relates to systems and methods for locating subsurface objects.
In this specification the expression subsurface object includes objects located below a land surface, below the surface of a body of water and below a seabed.
In a number of circumstances it is desirable to be able to determine the position of a subsurface object. Examples of such circumstances include the determination of the position of an underwater vehicle which, for example, is being used to inspect a pipeline and the determination of the position of a drilling tool during the directional drilling of an oil well.
Existing systems for determining the position of subsurface objects have disadvantages.
It is an object of the present invention to provide a system for determining the position of subsurface objects which alleviates at least some of the problems associated with existing systems.
According to one aspect of the present invention there is provided a subsurface object location system for determining the position of an object located below a surface, comprising a subsurface unit having transmitter means and being arranged to be mounted on the object, a plurality of surface units having receiver means and being arranged to be disposed at spaced locations at the surface, and processing means for determining the times of reception at the plurality of receiver means, of a signal transmitted from the transmitter means and determining the location of she object on the basis of the times of reception.
According to another aspect of the present invention there is provided a method for determining the position of an object located below a surface, comprising the steps of:
is mounting a subsurface unit having transmitter means on the object;
disposing a plurality of surface units having receiver means at spaced locations at the surface;
using the transmitter means to transmit a signal from the object;
determining the reception times of the signal at each of the plurality of receiver means; and determining the location of the object on the basis of the times of reception.
Preferably the processing means is arranged to compare the times of reception of the signals and to determine the location of the object on the basis of said comparison.
Said method preferably includes the steps of comparing the times of reception of the signals and determining the location of the object on the basis of said comparison.
Preferably at least three surface units are provided, more preferably still at least four surface units are provided.
In general there are four unknowns, namely three spatial co-ordinates and time. Not all of these unknowns need be determined using the reception times of the signals. For example depth may be determined by pressure sensors or other means. Further, or alternatively, a time reference may be provided at the transmitter. If all four unknowns have to be determined by time of reception, at least four surface units are required.
Subsurface, especially underwater, object location systems may use acoustic links. The use of acoustic links has disadvantages at least in part because of the multipath dispersion which occurs. Preferably the transmitter and receiver means are arranged respectively to transmit and receive inductive signals and in use the signals transmitted by the transmitter means are inductive signals. The use of inductive signals is advantageous because any multipath dispersion effects are minimized and the attenuation of the magnetic vector as electromagnetic radiation passes through water and many types of lithology is significantly lower than that of the electric vector.
Preferably a central unit is provided which is connected by a communication link to each of the surface units. The central unit may comprise a surface unit. Each surface unit may be arranged to relay signals to the central unit via the respective link. In some embodiments each surface unit is connected to the central unit by a high quality link having known characteristics. The central unit may be arranged to compare the time of reception by each surface unit, of signals transmitted from the subsurface unit. The central unit may be arranged to determine the time of reception of the signals by each surface unit on the basis of the time of reception of a relay signal from each surface unit and the known characteristics of the high quality links.
Each high quality link preferably has a high bandwidth. A high quality link is a link which has characteristics which can be determined accurately enough and remain within such tolerances that the time delay introduced by each link can be determined to an accuracy which allows the position of a subsurface object to be determined to the accuracy required in the particular circumstances. Such embodiments have the advantage that no master clock is required.
In other embodiments each surface unit is arranged to determine the time of reception of a signal from the subsurface unit by reference to a master clock and to relay an appropriate signal via the respective link to the central unit. The central unit may be arranged to determine the position of the subsurface object on the basis of the signals received from the surface units. The master clock may be that available via GPS. Such embodiments have the advantage that high quality links are not required.
The signals used to determine the position of the object may also be used to carry data. The subsurface unit may comprise receiver means for receiving signals from the surface. One or more surface unit and/or the central unit may comprise transmitter means for transmitting signals to the subsurface unit.
The object location system may be arranged to operate where the signals pass through earth/land and/or seawater.
The subsurface object location system may be a subsea object location system.